From the project lead's website Grimrukh's Nest :
Nightfall is a direct fan-made sequel to Dark Souls, with a new story, new characters and bosses, new world map (built mostly from the existing pieces), and much more. It’s basically a brand new Souls game built using the Dark Souls engine and assets.
Combat will be faster-paced, against a mixture of familiar and unfamiliar foes, with brand new voiced NPCs and side-quests.
Nightfall also takes many cues from the design of Majora’s Mask, one of my favorite games.